This invention relates to a method for laminating layers or articles, the method comprising curing a photopolymerizable composition through a colored, opaque, or reflective substrate. The article can be, for example, an electronic component, a printed circuit board, or the layers can be two opposing faces of a compact disc.
Photopolymerization of monomers using UV light to prepare adhesive compositions is an established part of polymer chemistry. Numerous photopolymerizable compositions, for example, those comprising ethylenically-unsaturated monomers and at least one photoinitiator, have been photopolymerized using UV irradiation. A necessary condition for these photopolymerizations is that the photopolymerizable compositions must be directly exposed to the UV irradiation in order for the photoinitiating component to generate the free radicals required to initiate the photopolymerization process. Numerous industrial processes rely on selective UV photopolymerization, wherein a mask is used to block UV irradiation to specified areas of a surface or substrate so that photopolymerization takes place only in the exposed areas.
In the electronics industry, numerous methods have been used to bond electronic components together for purposes of forming multilayer components or simply adhering a component to a substrate. Methods involving photopolymerization have generally been limited to situations where at least one of the substrate or the component is essentially transparent to UV irradiation. However, in the case of bonding components to a printed circuit board (PCB), this seldom is possible, since PCBs are often made of opaque and colored materials and they usually are at least partially covered with metallic circuit traces. In addition, the electronic component itself, e.g., a chip, typically is completely non-transmissive to electromagnetic radiation. In general, the electronics industry has turned to thermally polymerizable adhesives when reinforcement of solder bonds is desired. This is not entirely satisfactory because of the lengthy thermal curing cycles required. In addition, some electronics components may be heat sensitive and free-radical thermal polymerizations in general do not lend themselves to patterned or selective activation.
Photopolymerization has been used in the bonding of electronic components. For example, surfaces to be joined have been coated with a photopolymerizable adhesive, followed by irradiating the adhesive, then placing the two parts together and allowing the irradiated adhesive to completely cure. Alternatively, irradiation at the peripheral edges of parts to be joined can result in a bond of sufficient strength to temporarily hold a component in place. Often, heating the joined components is necessary for complete curing.
Curing of photopolymerizable adhesives by UV irradiation through a substrate is known. For example, DE 3939628 discloses bonding of electronic components to aluminum oxide or aluminum nitride ceramic substrates that are up to 1500 xcexcm thick by UV irradiation of at least 50 mW/cm output density. Transmission of UV light through an aluminum oxide ceramic substrate of 1016 xcexcm thickness is reported to be about 0.6%.
U.S. Pat. No. 4,656,314 discloses curing of a conductive metal-coated UV-curable ink on a translucent PCB by UV irradiation from both the top and bottom of the PCB, wherein at least some of the UV light passes through the substrate from the underside to assist in the complete curing of the ink. The substrate is characterized as a sheet of polyester or polycarbonate that must be at least partially translucent, preferably more than 50% translucent to UV light. Conventional print treated MYLAR (Dupont) is described as an effective commercially available substrate.
U.S. Pat. No. 5,065,505 discloses a method of connecting circuit boards wherein a photocurable adhesive is coated onto a light-transmissive circuit board on which electrodes have been formed. Light is irradiated through the circuit board from the side opposite the coated side, curing the adhesive in areas not shaded by the electrodes. Exemplified photoinitiators have absorption peak wavelengths ranging from 240 nm to 365 nm. Photoinitiators useful in the visible light range are not described. Suitable circuit board substrates include polyimide resin, polyester resin, and the like.
Japanese Kokai Application JP 7-106723 discloses curing of adhesives through a flexible circuit board that is transmissive to 5% or more UV light having a wavelength between 350 and 400 nm. Base films, through which UV-curing takes place, can include poly(etherimide), poly(ethersulfone), polyethylene naphthalate, polyether ether ketone, polycarbonate, and polyethylene terephthalate.
Japanese Examined Application JP 7-81114 discloses curing a photohardenable adhesive in the presence of a diketone photoinitiator and a dialkylamino benzophenone photosensitizer by irradiation through a semitransparent substrate using irradiation wavelengths up to 436 nm.
U.S. Pat. No. 5,607,985 discloses a photopolymerization initiator for visible-light polymerizing adhesives comprising a photopolymerization initiator, an aliphatic tertiary amine and a radical polymerizing monomer. Adhesion of a sandwich construction comprising two opaque glass pieces, each having 10% light transmissivity at 510 nm and 0% light transmissivity between 490 and 200 nm, on exposure for two minutes to a metal halide lamp, is described.
U.S. Pat. No. 5,798,015 discloses generating reactive species (adhesives) by providing a wavelength-specific sensitizer in association with a reactive species-generating photoinitiator and irradiating the wavelength-specific sensitizer. The method is used to laminate at least two layers together by coating the adhesive between the layers and irradiating to effect polymerization thereof, providing that at least one of the layers is a cellulosic or polyolefin nonwoven web or film and the sensitizer is one of a set of specified arylketoalkene moieties.
Optical recording discs, such as compact discs and CD-ROMs, often comprise two or more layers of a polymeric base substrate, each of which comprises a recording layer, bonded together by an adhesive with both recording layers facing each other. Typically, the recording layers comprise an opaque metal foil. Uniform curing of the adhesive between the foils is difficult. U.S. Pat. No. 5,360,652 discloses such an optical recording disc, wherein the adhesive is a photocurable adhesive. In order to adhere the two discs together, the recording medium is configured not to extend to the periphery of the discs so that UV irradiation rapidly cures the adhesive around the edges of the disc, allowing the masked adhesive under the recording medium to cure only by contact with initiators in the irradiated region.
U.S. Pat. No. 5,785,793 discloses one- or two-sided irradiation of an optical recording disc having one or two back-to-back storage medium layers. Curable adhesive is used on the side opposite from the recording medium, in either case, so that UV irradiation must pass through at least the recording medium in order to cure the adhesive. Heat management is an issue for optical recording disc manufacture, since the discs are easily warped and the recording medium is typically a low-melting metal, such as aluminum. Xenon flash lamps are preferred for irradiating the curable adhesive and bonding the discs together.
Bonding of DVD (Digital Versatile Disk) substrates is described by D. Skinner, xe2x80x9cUV Curing Through Semi-transparent Materialsxe2x80x94The Challenge of the DVD Bonding Process,xe2x80x9d RadCure Letter, April, 1998, p. 53-56, wherein UV light of 320-390 nm wavelengths is shown to penetrate a 40 nm thick coating of aluminum on a polycarbonate substrate with 91% transmissivity. Curing of an adhesive under these conditions is not disclosed.
Briefly, the present invention provides a method of laminating a structure comprising the steps of:
a) providing a structure comprising at least two layers and a photopolymerizable adhesive composition between the layers,
1) at least one of the layers being opaque or colored and transmissive to actinic radiation in an identified spectral region having one or more wavelengths greater than 400 nm and up to 1200 nm, the layer being essentially free of cellulosic and olefinic functionality,
2) the photopolymerizable composition comprising a photopolymerizable moiety and a photoinitiator therefor that absorbs actinic radiation in the identified spectral region of the radiation transmissive layer, the photopolymerizable moiety being polymerizable in a hydrosilation, cationic, or free radical polymerization process, with the proviso that the free radical polymerization process is free of dialkyaminobenzophenone sensitizer,
b) directing actinic radiation within the identified spectral region through the radiation transmissive layer and into the photopolymerizable composition for less than two minutes to cure the photopolymerizable composition,
whereby the resulting polymerized composition adheres to the layers and provides a laminated structure.
Optionally, the structure, including the photopolymerized composition, can be heated to complete the polymerization of the photopolymerizable composition.
Each of the layers of the invention independently can be a coating, a film, or a substrate, or it can be included in an article. Preferably the article can be an electrical component such as an integrated circuit chip (IC) or a printed circuit board (PCB).
In another aspect, there is provided a method for identifying suitable materials for two layers, a photopolymerizable adhesive composition, and a radiation source to be used for producing a laminated structure, the method comprising the steps of:
a) identifying two layers, at least one of which is colored, opaque, or reflective and is transmissive to actinic radiation in an identified spectral region having one or more wavelengths greater than 400 nm and up to 1200 nm, with the proviso that when the radiation transmission layer is colored or opaque, it is essentially free of cellulosic and olefinic functionality,
b) identifying a photopolymerizable composition to be disposed between the layers comprising a photopolymerizable moiety and a photoinitiator therefor that absorbs radiation in the identified spectral region of the radiation transmissive layer,
c) identifying a radiation source that provides actinic radiation in the identified spectral region of the radiation transmissive layer and in the light absorbing wavelengths of the photoinitiator,
whereby directing the actinic radiation, on demand, through the colored, opaque, or reflective layer, for a time sufficient to effect polymerization of the photopolymerizable composition, produces a laminated structure.
In a further aspect, the present invention provides a method for preparing a soldered and underfilled flip chip assembly on a circuit substrate, the method comprising the steps of:
a) providing
1) an integrated circuit chip having a surface comprising reflowable solder bumps with contact tips thereon, and
2) a printed circuit substrate having a bonding site,
wherein at least one of the chip and the circuit substrate is transmissive to actinic radiation in an identified spectral region having wavelengths greater than 400 nm and up to 1200 nm,
b) applying a photopolymerizable adhesive composition directly to one or both of the surface of the chip with solder bumps and the bonding site of the printed circuit substrate, the method providing exposure of the contact tips of the solder bumps of the chip, the photopolymerizable adhesive composition comprising a photopolymerizable moiety and a photoinitiator therefor, wherein the photopolymerizable composition absorbs radiation in the identified spectral region of the radiation transmissive chip or circuit substrate,
c) aligning and pressing the exposed tips of the bumps on the surface of the chip against the bonding site of the circuit substrate,
d) melting and reflowing the solder to establish electrical contact between the chip and the circuit substrate, wherein the photopolymerizable material remains substantially uncured, and
e) directing radiation within the identified spectral region through the radiation transmissive chip or circuit substrate for a time sufficient to cure the photopolymerizable adhesive composition and to produce the soldered and underfilled flip chip assembly on the circuit substrate.
Optionally, a functional evaluation of the soldered electrical connections may be performed prior to irradiation as described in step e). If the evaluation shows insufficient electrical contact between the chip and the substrate, the assembly can be reheated to allow easy removal of the chip from the substrate, after which the chip and the bonding site can be cleaned, soldering can be repeated, and a functional evaluation repeated to assure adequate electrical connection prior to irradiation.
In a still further aspect, the present invention provides a method of laminating a structure comprising the steps of
a) providing a structure comprising at least two layers and a photopolymerizable adhesive composition between the layers,
1) at least one of the layers being one or both of a reflective layer and a layer incorporated in an electronic component that is transmissive to actinic radiation in an identified spectral region having one or more wavelengths greater than 400 nm and up to 1200 nm,
2) the photopolymerizable composition comprising a photopolymerizable moiety and a photoinitiator therefor that absorbs actinic radiation in the identified spectral region of the radiation transmissive layer, the photopolymerizable moiety being polymerizable in a hydrosilation, cationic, or free radical polymerization process,
b) directing actinic radiation within the identified spectral region through the radiation transmissive layer and into the photopolymerizable composition for less than 2 minutes to cure the photopolymerizable composition,
whereby the resulting polymerized composition adheres to the layers and provides a laminated structure. In a preferred embodiment, this method provides a data storage disk.
In a yet further aspect, the present invention provides a method for preparing a soldered and underfilled flip chip assembly on a circuit substrate, the method comprising the steps of:
a) capillary underfilling a soldered chip on an integrated circuit substrate with a photopolymerizable adhesive composition comprising a photopolymerizble moiety and a photoinitiator therefor,
wherein the circuit board is transmissive to actinic radiation in an identified spectral region having wavelengths greater than 400 nm and up to 1200 nm, and
wherein the photopolymerizable composition absorbs radiation in the identified spectral region of the radiation transmissive circuit substrate, and
b) irradiating the circuit substrate from the side opposite the side bearing the soldered chip with actinic radiation in said identified spectral region.
In this application:
xe2x80x9cactinic radiationxe2x80x9d means photochemically active radiation and particle beams, including, but not limited to, accelerated particles, for example, electron beams; and electromagnetic radiation, for example, microwaves, infrared radiation, visible light, ultraviolet light, X-rays, and gamma-rays;
xe2x80x9ccurexe2x80x9d and xe2x80x9cpolymerizexe2x80x9d are used interchangeably in this application to indicate a chemical reaction in which relatively simple molecules combine to form a chain or net-like macromolecule;
xe2x80x9ccoloredxe2x80x9d means having visually perceptible color either to the naked or unaided eye, or to the aided eye, e.g., the light visually perceived by the eye after shining a light on a substrate;
xe2x80x9cDVDxe2x80x9d means both Digital Video Disc and Digital Versatile Disc;
xe2x80x9cUVxe2x80x9d or xe2x80x9cultravioletxe2x80x9d means actinic radiation having a spectral output between about 200 and about 400 nanometers;
xe2x80x9cvisible lightxe2x80x9d means actinic radiation having a spectral output greater than about 400 to about 700 nanometers;
xe2x80x9cnear IRxe2x80x9d or xe2x80x9cnear infraredxe2x80x9d means actinic radiation having a spectral output between about 700 and about 1200 nanometers;
xe2x80x9ctransparentxe2x80x9d means that the material, when viewed under an optical microscope, (e.g., with a stereoscopic microscope at 50xc3x97 and under oblique or transmitted light), has the property of transmitting rays of visible light so that images of articles viewed through the material have sharp edges;
xe2x80x9ctranslucentxe2x80x9d means that the material, in whole or in part (asd when patterned), when viewed as described under an optical microscope, has the property of transmitting visible light to some degree so that images have unclear or blurred edges;
xe2x80x9copaquexe2x80x9d means that the material, when viewed as described under an optical microscope, has the property of being impervious to radiation at a given wavelength, in a multilayered material, one or more layers have continuous or discontinuous opaque regions; curing takes place through the opaque regions;
xe2x80x9clight transmissivexe2x80x9d means a substrate or article having an optical density of 4.0 or less, preferably an optical density between 4.0 and 2.0, and more preferably having an optical density between 4.0 and 3.0 when irradiated with a light of wavelength greater than 400 to 1200 nanometers;
xe2x80x9creflectivexe2x80x9d means capable of bending or returning or throwing back at least 90% of the incident light from a surface irradiated by that light;
xe2x80x9cgroupxe2x80x9d or xe2x80x9ccompoundxe2x80x9d or xe2x80x9cligandxe2x80x9d means a chemical species that allows for substitution or which may be substituted by conventional substituents which do not interfere with the desired product, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.;
xe2x80x9cphotopolymerizable moietyxe2x80x9d means a photopolymerizable monomer, oligomer, or polymer; and
xe2x80x9creflowxe2x80x9d means melting of a solder.
The present invention is advantageous in that it provides a unique method for laminating layers or articles using a photopolymerizable adhesive composition. The adhesive can be cured on demand indirectly through a variety of colored, opaque, or reflective substrates using actinic radiation in the range of greater than 400 to 1200 nm.
The curing can take place at elevated or ambient (20-25xc2x0 C.) temperatures, which can be advantageous for temperature sensitive substrates.
Conventional methods of adhering face-down integrated circuits (e.g., flip-chips), or other articles, to ceramic, polyimide or epoxy-filled printed circuit boards (PCB""s), or other substrates, typically use thermally cured epoxy-based adhesives which can be applied with capillary action after a solder connection has already been established, when reinforcement is necessary. This becomes increasingly difficult as, for example, chip size increases and as the number of electrical connections per chip increases. Also, there is a need in the art for pre-applied low temperature, e.g., less than 50xc2x0 C., cure-on-demand adhesives and that do not use UV activated systems. UV activated systems can be impractical due to their inability to transmit UV light either edgewise or through an epoxy PCB or other substrate. Prior art UV methods traditionally required colorless transparent substrates. The present invention method, comprising photocuring using visible or near-infrared light, is advantageous in that colored and opaque substrates can be used. It is advantageous, also, that UV absorbing or reflecting substrates that are transmissive to actinic radiation within the desired spectral range can be used in the present invention method. Further, it is desirable to prepare wafers or circuits having pre-applied, self-fluxing, curable adhesives applied to the underside (e.g., bumped side) of the chip or circuit. Also, rapid cure of the adhesive is desired in order to minimize production time.
The present invention overcomes deficiencies of conventional curing systems by providing a method to rapidly photopolymerize a variety of radiation curable compositions (both thermoplastic films and flowable resins) directly through various PCB""s or other substrates, both in unmetallized and metallized regions of the PCB""s or other substrates, using visible light and/or near infrared curing systems. The ability to cure through the back side of a circuit substrate depends on several factors including line width and spacing, angle of irradiation, substrate and circuit thickness, and the type of metallized tracing. The low temperature method of the present invention can be useful with temperature sensitive substrates. Any composition that can be polymerized or crosslinked via direct photolysis or indirect photochemical generation of a thermal catalyst or initiator can be used in this approach. Shelf-stable, premixed compositions can be cured on demand using the method of the present invention.
The present invention is also advantageous in that it allows the use of a variety of light sources for initiating photopolymerization reactions, including many conventional, non-specialized light sources, such as halogen bulbs or other sources of visible light and/or near infrared radiation. The method of the invention is thus inherently safer to a human user than methods using, e.g., ultraviolet light, since less visible light and/or near infrared radiation is required to produce an equivalent amount of light density. The use of visible light and/or near infrared radiation sources is also advantageous over ultraviolet light sources because of the known degradative effects of UV light on substrates such as polymers. Finally, the invention allows facile visible-light curing of photopolymerizable compositions comprising helpful ultraviolet absorbing additives such as UV stabilizers and antioxidants.
U.S. application Ser. No. 08/986,661, filed Dec. 8, 1997, which is incorporated herein by reference for its disclosure of flip-chip assemblies and processes including all Figs. of the Drawing, relates to a method and assembly for connecting an integrated circuit chip to a circuit substrate.